AP BIOLOGY

Detailed explanation-1: -Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer.

Detailed explanation-2: -The plasma membrane is virtually impermeable against larger, uncharged polar molecules and all charged molecules including ions.

Detailed explanation-3: -Facilitated diffusion therefore allows polar and charged molecules, such as carbohydrates, amino acids, nucleosides, and ions, to cross the plasma membrane. Two classes of proteins that mediate facilitated diffusion are generally distinguished: carrier proteins and channel proteins.

Detailed explanation-4: -Only the smallest molecules like water, carbon dioxide, and oxygen can freely diffuse across cell membranes.